Abstract EANA2025-149

Understanding the origin of life requires exploring the dynamic interplay between geochemistry and prebiotic organic chemistry. In this context, hydrogen cyanide (HCN) stands out as a versatile molecular precursor, while sulphur-bearing minerals are increasingly recognised as active agents shaping early molecular evolution. This study investigates the influence of mineral substrates on cyanide polymerisation under alkaline aqueous aerosol conditions, simulating plausible scenarios on early Earth and Mars.

The general objective was to determine whether specific minerals—pyrite (FeS₂) and magnesium sulphate (MgSO₄)—can modulate molecular complexity during cyanide polymerisation, thereby contributing to the formation of organic macromolecules relevant to prebiotic chemistry.

Two series of biphasic inorganic-organic systems were synthesised by reacting ammonium cyanide (formed from NaCN + NH4Cl) in alkaline aqueous aerosols with either pyrite nanoparticles (for PY-OM series) or powdered magnesium sulphate (for SU-OM series) over four and twenty-one days. The products were characterised using a comprehensive suite of techniques, including transmission electron microscopy (TEM), scanning transmission electron microscopy with energy-dispersive X-ray spectroscopy (STEM-EDXS), Fourier-transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), and thermal analysis (TG-DTG-DSC).

The PY-OM systems revealed cyanide-derived polymers uniformly coating pyrite particles, stabilising the mineral and suppressing sulphate signals (observed in the commercial pyrite used) in final products. In contrast, the SU-OM systems underwent substantial mineralogical transformations. Initially, magnesium sulphate formed brucite (Mg(OH)₂) (SU-OM4). This brucite then reacted with silicate (sourced from borosilicate flasks under high pH) to yield magnesium silicate hydrates (M-S-H) (SU-OM21). These transformations were confirmed by FT-IR and XRD analyses. In addition, thermal analysis of the PY-OM and SU-OM series indicated increased organic content with reaction time, including the formation of refractory organic material. Both systems produced organic-inorganic composites, with SU-OM21 showing particularly strong hybrid character.

To further probe the role of sulphate species, additional polymerisation experiments were performed using ferric and ferrous sulphates. These reactions did not result in sulphate reduction, but rather in the formation of soluble ferrocyanide complex salts. Our results suggest that in alkaline, cyanide-rich conditions, Fe³⁺ is reduced to Fe²⁺—likely mediated by diaminomaleonitrile—allowing the generation of sodium ferrocyanide, while sulphates remain in solution. This iron complex salt may have functioned as reservoir for cyanide in prebiotic aqueous environments, with potential implications for chemical evolution on early planetary surfaces.

These findings advance our understanding of prebiotic chemistry by demonstrating that mineral surfaces not only stabilise organic matter but also induce structural and chemical changes that promote (macro)molecular diversification. The transformation of mineral phases, such as the formation of brucite and M-S-H, also creates reactive interfaces with catalytic and compartmentalisation potential, possibly favouring the emergence of proto-metabolic systems. The observed processes align with an extend current cyanosulfidic scenario, showing that sulphur-bearing mineral interfaces in alkaline, cyanide-rich aerosols can drive the structural evolution of both organic and inorganic phases. This offers a plausible geochemical framework for the emergence of molecular complexity on early Earth and Mars (1).

1. López-García et al. Science Advances 2025 (under review).